Field of the Invention
The present invention is directed to systems, methods, rotating machines (as well as power electronic converters associated with the rotating machines) and computer program products that relate to electric power that is applied to an electric power grid after being generated from renewable power generation facilities (xe2x80x9crenewablesxe2x80x9d). More specifically, the present invention is directed to systems, methods, rotating machines and computer program products for enhancing electric power produced by renewable facilities, such as wind turbine plants, solar electric power plants and the like, so as to make electric power generated thereby as commercially valuable and fungible as electric power produced by traditional sources, e.g., fossil fuel power plants, hydroelectric plants, nuclear plants and the like.
Renewables are xe2x80x9cnaturalxe2x80x9d power production sources that instinctively should be regarded as optimal sources of energy for producing electric power. Renewables do not require burning of fossil fuels, do not result in nuclear waste by-products, do not require the channeling of water sources, and do not otherwise disturb the environment.
On the other hand, renewables are burdened by xe2x80x9cweaknessxe2x80x9d and xe2x80x9cvariabilityxe2x80x9d (each defined below), thus not offering AC power grid operators the (voltage) xe2x80x9cstiffnessxe2x80x9d and type of planned control that the power grid operators and owners possess with conventional power generation facilities, especially during normal operation as well as during faults. The term xe2x80x9cweaknessxe2x80x9d refers to a condition or quality of being weak. Moreover, in the context of the present document, weakness in reference to a power grid refers to (1) a risk of failure when subject to pressure, stress or strain, (2) having a lack of physical strength, energy or vigor, (3) having a lack of proper strength for resisting failure in a strained environment, (4) having a lack of ability to function normally or fully, and (5) having a lack of aptitude or skill. The term xe2x80x9cvariabilityxe2x80x9d in reference to a power surge relates to (1) a quality, state or degree of being variable or changeable, (2) having characteristics that are subject to vary or have a tendency to variation, (3) have a characteristic (such as wind or currents) tending to change direction, and (4) being capricious (irregular and unpredictable).
xe2x80x9cStiffness,xe2x80x9d (defined below) in the context of voltage in a power grid scenario relates to (1) corresponding to an infinite bus in a power grid, (2) having a quality, state or degree of being difficult to change or disturb, (3) being firm as in purpose, or resolute, and (4) being potent or strong.
As recognized by the present inventors, the AC power grid is not yet exposed to full stress regarding the weakness associated with renewable power generation facilities. Such facilities have not reached sufficient penetration in the grid, as compared to the existing methods and mechanisms known for supporting stable power grid operation and for proper fault handling. An increase in percentage of conventional renewable facilities would tend to limit the transmission availability and capability of existing power grids. Existing methods and mechanisms for stable power grid operation and fault handling are based on the fact that the existing power plants, converting energy from the traditional sources into electrical power, possess a transient built-in overload capability, especially regarding reactive power and fault currents. Consequently, modem AC power grids exhibit voltage stiffness sufficient for normal operation. This enable the grid to be operated within a few percent voltage variation around its nominal value.
By way of background, reactive power is used to stabilize the AC power grid""s operation and its voltage stiffness and thus the grid""s power quality. Fault currents are considerably larger than the nominal current levels, say 3 to 10 times, which ensure proper and rapid fault handling with existing methods and mechanisms.
Modern designers of electric conversion systems embedded in renewable facilities have tried to identify ways to use the same conversion systems to provide both active and reactive power from renewable facilities with a power throughput through only one type of power converter. Some designers thus mainly focus on the reactive power issue during normal operation and the associated perpetual AC voltage control in the operative AC power grid. However, they neglect the transient fault current and the transient voltage stiffness issues, thus leaving transient needs to be solved by others, such as AC power grid owners and operators and traditional AC power plant owners. Thus, traditional renewables may be environmentally friendly, but the nature of the power they produce and the strain they place on the system is viewed as a burden on the power production system, if deployed in great numbers. When attempting to address the power quality issue, renewable designers sometimes rely on a solution that includes expensively dimensioned power semiconductor hardware when the renewable facility has the duty to support, or even maintain, the voltage stiffness and/or the supply of short circuit power during faults.
Short-circuit Power, Voltage Stiffness
Short-circuit Capacity, SCC
A product of the pre-fault bus voltage and the post-fault current is referred to as short-circuit capacity (SCC), or fault level of a bus in question. By definition it is the value
SCC=EI=E*E/X 
where E is bus voltage magnitude and X is reactance for the Thevenin circuit.
SCC and voltage stiffness
As E (pre-fault bus voltage, in the above equation) equals the pre-fault voltage of the shorted bus and this voltage normally has a magnitude of approximately 1 pu, (p.u.=per unit) the SCC becomes
SCC=(approximately)1/X 
(where X is the Thevenin reactance; same as X above)
The SCC has a direct bearing on the choice of circuit breakers, which must have an interrupt megavoltampere capacity equalling at least the fault level value for the bus in question. It should be noted that there is a direct relationship between the SCC and the xe2x80x98voltage stiffnessxe2x80x99 of a system.
AC/DC System Interconnectionxe2x80x94Short-circuit Ratio, SCR
Since the AC power grid""s system strength has an important impact in the AC/DC system interconnection, it is useful to have a simple way of measuring and comparing relative strength of AC systems. The short-circuit ratio (SCR) has evolved as such a measure. It is defined as:
SCR=(short-circuit MVA of AC system)/(DC converter MW rating). 
The operation of a DC system when connected to a weak (low short-circuit capacity) AC system is, as an example, associated with the following problems:
(1) high dynamic over-voltages, (2) voltage instability, (3) harmonic resonance, and (4) objectionable voltage flicker.
Stability Improvement
Practically, useful methods to improve power system (transient) stability include:
Increase of system voltage;
Reduction of transfer reactance; and
Use of high-speed circuit breakers and re-closing breakers.
Rotating Machine and Power Grid Interaction
To install a rotating machine and for a rotating machine drive system to function well, it is of importance to carefully study interconnection issues with the network. Voltage levels, short-circuit levels (capacity), type of network (distribution or industry), connected phase compensation equipment, disturbances (e.g. lightning), interruption frequency, etc. are to be studied.
An issue to address when installing new process equipment in industry is how large the installed rotating machine can be chosen (installed) without negatively effecting the voltage quality, especially if it is a motor that is started frequently and thereby behaving like a short-circuited rotating machine. Since a short-circuited rotating machine will result in a voltage drop at the point of connection, as well as in the area of connection, a maximum size of a rotating machine to be connected is determined by a maximum allowable voltage drop (power quality requirement) during transients, such as when starting a rotating machine as a motor. These voltage variations can be difficult to handle for other load objects in the vicinity of the rotating machine, especially if it is a motor started frequently.
Short-circuit Limiters
The purpose of a short-circuit (fault) current limiting device is to limit both impulse and fundamental frequency fault currents. When connecting large AC machines to high-power grids, with low damping, these devices are also used to create fault current""s zero-crossings to simplify the interruption of fault currents.
Short-Circuit Level Tradeoffs
When a load is connected to the bus where a fault will occur, the associated voltage drop on the bus is directly proportional to the Thxc3xa9venin reactance, X. A perfectly xe2x80x98voltage stiffxe2x80x99 (or often called xe2x80x98infinitexe2x80x99) bus requires that X=0. Such a bus will have an infinite fault level. Consequently, there are conflicting requirements concerning short-circuit levels. The fault level further increases with the system voltage.
It is not only high short-circuit powers that can cause problems. Too low of a short-circuit power can be a problem for power quality (PQ). Hence, there is a conflicting situation where short-circuit powers should be kept low to avoid high short-circuit currents and associated costs, while at the same time it should be high to reduce power quality problems.
As explained in xe2x80x9cModem Power System Analysisxe2x80x9d I. J. Nagrath and D. P. Kothari, Tata McGraw-Hill Publishing Company, New Delhi, 1980 (page 441), xe2x80x9c[R]ecent trends in design of large alternators tend towards lower short-circuit ratio (SCR=1/Xd) which is achieved by reducing machine air gap with consequent savings in machine mmf, weight and cost. Reduction in the size of rotor reduces inertia constant lowering thereby the stability margin. The loss in stability margin is made up by such features as lower reactance lines, faster circuit breakers and faster excitation systems as discussed already, and faster system valving to be discussed later in this article.xe2x80x9d
U.S. Pat. No. 5,225,712, incorporated herein by reference is an example of how conventional power system designers, such as the case in windmills, focus on a normal operation reactive power issue, not a failed mode operation issue. U.S. Pat. No. 5,225,712 uses a self-commutated inverter with xe2x80x9cactive switchesxe2x80x9d to convert active power from an intermediate DC to standard frequency AC. At the same time, the inverter adds reactive power. However, as recognized by the present inventors, the number of power semiconductors for full fault current handling, and thus voltage stiffness, is a factor 10 to 30 larger than a most lean embodiment of the invention disclosed herein.
The high number of power semiconductors that implement the active switches is essentially determined by growing fault current demands from power grid operators on renewables facility owners and is identified as a problem by the present inventors. Regarding fault conditions and aiming at fault currents 3 to 10 times the nominal values, there is an almost instantaneous thermal response with a temperature rise above the design values, say 150xc2x0 C., allowed in the inverter""s Si-based active switches, available within the foreseeable future. SiC-based active switches allowing, say 300-500xc2x0 C., are not expected to become available within a decade or so and furthermore are likely to be limited by a similar inherent instantaneous thermal response because the switches will be built with a lesser amount of active material.
So, there is a long-term cost-effectiveness problem associated with allowing fault currents 3 to 10 times the nominal current because the inverter""s cost will increase by hundreds of percent to tolerate such fault currents, even if the high current level exist, only for some ten milliseconds or so.
With a growing share of renewables facilities, like wind power facilities, in the future connected to the power grid for power generation, there will thus be a problem of not only supplying reactive power during normal power grid operation, but also supplying considerable short-term over-currents for fault handling to avoid voltage sags and to keep voltage stiffness in the power grid. This is because most existing, and especially, most commonly implemented schemes for system protection are based on fault currents considerably larger than the nominal current levels, say 3 to 10 times.
Power semiconductor converters like self-commutated inverters as well as mains- or machine-commutated converters are defective, or at least ineffectively, due to their production of harmonics and sensitivity to voltage sags as well as to unbalanced three-phase voltages in the AC power grids; all of these drawbacks can be seen as xe2x80x9chinderingxe2x80x9d side-effects.
To address the variability issue above, even the early pioneers of renewables like wind power attempted to identify ways to xe2x80x9cstorexe2x80x9d wind generated electric power in times of excess, so as to later compensate for time periods when there are lulls in the wind. For example, Poul La Cour (1846-1908) from Denmark, was one of the early pioneers in wind generated electricity. Poul La Cour built the world""s first electricity generating wind turbine in 1891. This design included DC generators and stored energy as hydrogen. Poul La Cour was concerned with the storage of energy because he used the electricity from his wind turbines for electrolysis in order to produce hydrogen for the gas lights in his school.
This concept of energy storage has not been abandoned and even modem inventors of wind turbine electric generation facilities are still trying to identify ways to use physical media to store the energy produced by windmills. See, e.g., U.S. Pat. No. 5,225,712, which uses fuel cells, batteries, and the like as physical media to store electrical power, and, which embodies similar self-commutated units to combine active power from both the windmill""s generator-rectifier combination and the embedded fuel cells, batteries, and the like to form a power output via only one power converter from an intermediate DC link to the standard frequency AC power grid.
In the early days, wind energy plants were generally isolated from one another and provided small scale generation facilities. Through a variety of experiments, wind energy plants have generally evolved and a common theme is now to group a number of wind turbines together so as to form wind farms that can generate up to tens of megawatts via the aggregation of smaller plants that produce only slightly above one megawatt each. Most modem rotor blades on large wind turbines are made of glass fiber reinforced plastics (GRP). These wind power plants are today planned to grow slightly above 3 or 5 megawatts per unit, limited by a reliable and transportable blade size of the wind turbine, (the xe2x80x9cpropellerxe2x80x9d).
Solar electricity plants are to a growing extent based on photovoltaics, PV, where power electronic converters feed electricity from solar cells into the power grid. Another solution is based on solar heaters with boilers and turbines driving electrical power generators connected to the grid. Both solutions can be embodied with energy storage to address the variability issue. Boiler-based systems can store energy in steam or molten salt as examples. Embodied with turbo power generators, they can behave more or less like a fossil-fueled steam power plant with fairly limited problems regarding weakness, voltage stiffness and variability. PV-based systems face problems, regarding weakness, voltage stiffness and variability, which are similar to wind power plants and solutions like embedded fuel cells, batteries, and the like. PV-based systems often form a power output via only one power converter from an intermediate DC link to the standard frequency AC power grid.
Drawbacks of Wind Power Systems as an Example of Renewable Power Facilities
The invention is focused on large-scale electrical generation with electrical connections to the electrical power grid. So, the characteristics of stand-alone systems are omitted in the present discussion. Existing wind power plants are based on a continuous evolution since the 1970s with growing power ratings as well as growing numbers installed today, summing up to  greater than 4 GWpeak/year and with a growth of  greater than 35%/year. The plants are designed as small-scale units around the fact that minor energy contributions to the grid are not jeopardizing the performance of the power grid, which is to more than 90% supplied by the large-scale units based on coal, oil, gas, nuclear and hydro. The existing wind power plants and their direct power grid adaptation, i.e., their almost only mill-by-mill AC connection, are furthermore used because there have been no better alternatives. The published literature on conventional wind power systems is a catalog of patch-work measures to remedy this fatal flaw, see, e.g., Ackerman, T., et al., xe2x80x9cWind Energy Technology and Current Status: A Reviewxe2x80x9d, Renewable and Sustainable Energy Reviews 4(2000) 315-374; and Soder, L., xe2x80x9cThe Operation Valve of Wind Power in the Deregulation Swedish Marketxe2x80x9d, First International Workshop on Feasibility of HVDC Transmission Networks for Offshore Wind Farms, Mar. 30-31, 2001. Also see, e.g., an interview with Eltra""s CEO, Georg Styrbro in ERA (a Swedish Magazine) No. 4, 2001 (pp 40-42), roughly translated as, xe2x80x9cIf the wind becomes strong instead of light or if the expected storm arrives several hours before estimated, then we stand there with a lot of wind power, which we have not been able to sell over the power exchange earlier during the day. If the storm suddenly passes by Denmark or instead becomes a light wind, we could end up in the opposite situation. Then we might have promised away too much power. Then we must buy or sell [power] on the balance power market, and that of course becomes expensive, he says.xe2x80x9d
The following three lists include some specific examples:
For Stand-alone Windmills for Electrical Power Generation:
1) with an asynchronous machine which acts as a generator, but inherently consumes reactive power from the AC grid;
2) compensated by a fixed capacitor bank to a reasonable power factor; and 3) with a risk for so called xe2x80x9cisland operation,xe2x80x9d xe2x80x9cmagnetizedxe2x80x9d by the capacitor bank with a frequency differing with tens of Hz from the nominal value after faults;
For Wind Power Plants Erected with a Fixed Speed Adaptation Between the Wind Turbine and the Electrical Generator:
1) embodied as a mechanical gear-box to increase the speed of the generator shaft;
2) at a cost which is 3 to 5 times the cost of the generator;
3) at a large reduction of the mean time between faults, MTBF; and
4) at a large increase in the mean time to repair, MTTR;
The Power Quality Aspects at the Point of Grid Connection Have been Addressed Lately:
1) the tower shadow is a low-frequency, periodic disturbance which is a cause of xe2x80x9cflicker,xe2x80x9d a low-frequency variation of the grid voltage causing low-frequency RMS-value variation, e.g., an inconstant or wavering light, etc.; and
2) the stochastic character of the wind energy causes also flicker, both aspects compensated by static-var-compensators, SVCs or local energy storage units;
Adjustable-speed wind power plants use the wind energy better than a constant-speed one. They include e.g., a constant DC voltage link frequency converter between a variable-frequency power generator driven by a variable-speed wind turbine and the power grid with a standard frequency AC:
1) The gear-box is kept because the asynchronous machine, i.e., the industrial work-horse normally used as a motor, can be used as the least expensive type of generator;
2) The gear-box is redundant when using multi-pole low-frequency generators necessarily designed with many poles and made with very large diameters;
3) Power quality and power factor issues are partly eliminated by inherent capabilities during steady-state operation with low fluctuations in the wind speed;
4) Energy storage devices, based on hydrogen and fuel cells or electrochemical accumulator batteries or the like with a very high cost (e.g., 3 to 10 SEK/kWh for a system with fuel cells, accumulators, etc., compared to 0.03 to 0.3 SEK/kWh for bulk power from large-scale generation based on coal, oil, gas, bio-fuel, nuclear and hydro) per kWh compared to sales price are sometimes used at the DC voltage link inside some wind energy plants to balance out power fluctuations like wind gusts;
5) Reactive power production capability towards the power grid is normally inherent in large synchronous generators and can be partly incorporated in a wind mill adjustable-speed plant, although it is heavily limited by operating close to rated current during fault conditions. In conventional systems, such as that described in U.S. Pat. No. 5,083,039 (incorporated herein by reference) and U.S. Pat. No. 5,225,712 that include pulse-width modulation (PWM) inverters, mechanisms are sought for providing reactive power to the grid. This is important because reactive power is important for operating an AC power grid.
U.S. Pat. No. 4,941,097 (incorporated herein by reference) explains that PWM converters are not encumbered by long time constants associated with speed governors and thus may be expected to surpass the performance of AC generator stations in providing dynamic enhancement in the utility system. On the other hand, the present inventors have recognized that rotating electric machines, like generators and compensators, possess an inherent overload capability, which is superior to all power electronics especially PWM insulated gate bipolar transistor (IGBT) inverters, with very limited overload capability. Furthermore, conventional power plants (coal, oil, gas, bio-fuel, nuclear and hydro) produce fault currents 5 to 10 times the nominal current, a fact on which rapid fault clearing is based in the existing protection schemes of the main AC power grid.
There are also mills with gear-boxes, wound rotor asynchronous machines and speed variation based on rotor resistance variation (see, e.g., Opti-Slip(copyright) from Vestas A/S, DK, xe2x80x9cSemi-variable speed operationxe2x80x94a compromise?xe2x80x9d, described in Proceedings of 17 Annual Conference, British Wind Energy Association. Jul. 19-21, 1995, Warwick, UK) as well as on rotor converter cascades (see, e.g., PCT Publication WO99/07996), i.e., adjustable-speed systems which do not need fully rated power converters to be connected to the power grid.
Recently installed wind power plants have been erected as wind farms with several, constant-speed and/or variable-speed units connected to the same point in the electric power distribution grid, which:
1) simplifies power quality issues like the remedial use of SVCs mentioned above,
2) simplifies maintenance, and
3) simplifies operation, but has not simplified the power grid""s start-up procedures, maintenance, fault handling based on large short-circuit power, etc.
Sea-based wind farms have recently been commercialized. Several of these are equipped with AC-to-AC converters inside, i.e., the land-based technology. The inherent capacitance in the power cable and its reactive power generation limits the transmission cable length at AC standard frequencies (50 or 60 Hz). DC transmission is proposed (see, e.g., PCT Publication WO97/45908, incorporated herein by reference) for cables from a wind farm to the grid.
When conventional wind power systems are connected in an AC multi-terminal configuration to the power grid, patch-work measures have had to be devised to make them have the appearance of a xe2x80x9cnegative loadxe2x80x9d, more precisely a negative admittance, with proper steady-state active and reactive behavior and being xe2x80x9charmlessxe2x80x9d during major faults in the power grid. Finally, there is no delivery guarantee on electric energy trade of forward contracts, as an actor in the business, without adding/forcing the use of very expensive energy storage devices, based on hydrogen and fuel cells or electrochemical accumulator batteries or the like, which also must be considered as patch-work measures.
The background of the invention can be summarized by reciting some general problems, recognized by the present inventors, associated with renewables deployed on a large scale and fed into AC power grids as follows.
(a) Throughput of active power, P, is a problem from renewable energy sources like wind turbine(s) to an AC power grid. It is well-known to one skilled in the power systems art that there are mainly reactive characteristics that describe the normal operation of all AC HV and MV power grids. So, it is important to handle a suitable value of reactive power, Q, to form an AC complex power combination P+jQ to operate the power in-feed properly, either.
(a1) with a utility-demanded power factor, or
(a2) with a voltage control capacity.
(b) Reactive power compensation so that the power factor equals zero, during normal operation, is a utility-demand for the major part, of existing installations for voltage stiffness.
(c) Another problem is short circuit power capability during faults like short circuits and/or faults to ground. This power capability is mainly reactive and provides fault handling and voltage stiffness.
(d) Another problem is additional energy supply capability from a rotating electrical machine, preferably driven by a prime mover, P.M.,
(d1) to supply a part of a failing energyxe2x80x94compared to the prognosticated and sold energy-during normal operation, and/or
(d2) to supply energy for normally rare start-up procedures especially black-grid start.
(e) Another problem is energy storage capability to eliminate voltage flicker due to xe2x80x9cshadowsxe2x80x9d, like e.g., tower shadow and wind gusts during normal operation of wind power plants, and like e.g., shadows caused by clouds.
(f) Another problem is, later referred to as a problem of xe2x80x9ctype (f)xe2x80x9d is energy storage capability during faults where the voltage sags and the transferable power capability from wind to grid is temporarily reduced to maybe as low values as 5 to 10% of nominal value during fault time of some 0.2 seconds (a standard power plant criterion).
(g) Right-of ways for transmission lines are not easy to achieve because the visual impact, as well as the levels of questioned, low-frequency, LF, electro-magnetic fields, EMFs, are considerable from three-phase AC overhead lines.
Problems (a), (b) and (e) are fairly well known demands and problems, while problems (c), (d) and (f) have been recognized by the present inventors as being market hindrances for renewable facilities. Problems (c), (d) and (f) will primarily arise due to the increased power ratingsxe2x80x94of both single facilities, like wind mills and wind parks as well as solar electric power generation plantsxe2x80x94emanating from recent RandD steps in renewables power generation. Problems (c), (d) and (f) will secondarily arise due to the increased number of installations, too.
Techniques to solve problem (d1) will likely be limited in use to fairly small energy amounts on a yearly base due to the availability of virtual energy storage, to prime power from renewables, as discussed in U.S. patent application Ser. No. 09/749,999 into premier power, i.e., to overcome the main problem of xe2x80x9cvariabilityxe2x80x9d. To solve problem (d2) a similar source of energy is needed to supply energy for start-up procedures, especially for start-up on black grids.
Problems of type (f) have hitherto been solved by tripping a connectivity to the renewable power plants but can, as presently recognized, be solved by using the physical arrangementsxe2x80x94wind turbines, rotating electrical machines, power electronic converters, etc. xe2x80x94twice, first for power conversion and second for energy storage during 0.02 to about 2 seconds.
An Overview of Rotating AC Machines
Types of Conventional Rotating AC Machines:
Synchronous machines, SM, can generate and consume reactive power as well as active power, whereby they are called upon to be operated as generators or motors, depending on their production or consumption of active power. SMs also have an imbedded capability to store energy in their moment of inertia, J, often associated with the technical term inertia constant, H. It is known to one skilled in the power generation art that SMs follow a so-called swing equation.
Synchronous compensators (condensers) (i.e., a special type of machine in the family of synchronous machines, SM) do not generate active power, but only absorb or generate reactive power to control the voltage magnitude in the power system. Synchronous condensers can also store energy in their moment of inertia, J, often associated with the technical term xe2x80x9cinertia constant,xe2x80x9d H. It is known that for synchronous compensators the swing equation""s Pmech=0.
Asynchronous machines, AM, can generate and consume active power, but consume only reactive power, and are called to be operated as generators or motors depending on their production or consumption of active power. An AM also stores energy in its moment of inertia, J, often associated with a technical term xe2x80x9cstart-up time constantxe2x80x9d, but an AM is loosely connected to the grid frequency due to the asynchronous principle.
As discussed in WO 00/67363, the contents of which being incorporated herein by reference, there is a new type of rotating AC machine. The present inventors have recognized that when used with an intelligent controller, this new type of rotating AC machine enables renewable-based power production facilities to make the power from which as fungible and desirable as power produced from conventional power production facilities, without being a detriment from a grid operator""s perspective.
As will be discussed in more detail with regard to the detailed description of the present invention, this rotating AC machine is coupled in various ways to an intelligent coactive converter""s output line. The machine itself is a constant frequency output rotating AC machine, xM, with variable speed. It mainly acts as a reactive compensator for xe2x80x9cstiffnessxe2x80x9d and an active compensator for reduced xe2x80x9cvariabilityxe2x80x9d 1) based on the moment of inertia in the rotating parts and 2) optionally based on a prime mover, P.M. The present invention fully leverages these attributes to enable renewable facilities to reliably deliver power to the power grid and to avoid stiffness and variability issues typically associated with renewable-based power production facilities.
xe2x80x9cyMxe2x80x9d is the nomenclature given to constant-frequency output, rotating AC machines optionally used in embodiments of the present invention:
yMs comprise xMs, with a variable/varying speed capability, and SMs, with constant speed capability, coupled in various ways to an intelligent coactive converter""s output line, as will be discussed, and acting as a reactive compensator for xe2x80x9cstiffnessxe2x80x9d and an active compensator for reduced xe2x80x9cvariabilityxe2x80x9d 1) based on the moment of inertia in the rotating parts and 2) optionally based on a prime mover, P.M.
Several types of variable or constant speed, rotating AC machines are furthermore used as various alternatives in different embodiments of the present invention:
SM, AM or xM are types of rotating electrical AC machines that are mechanically coupled to the wind turbines (or other renewable power generation facilities) and mainly driven as generators by the wind turbines. They can be idling or motoring during transient conditions like faults or start-up procedures.
The following is a further description of some differences between an xM and an SM. An SM has, under stationary operational conditions, a fixed relation between the mechanical rotational speed and the output frequency of the rotating AC machine, while this is not the case for an xM. The SM can hence be described as a constant speed rotating AC machine, while xM can be described as a rotating AC machine with variable/varying speed capability, where the mechanical speeds of the AC machines are related to the AC network frequency.
Sources and Sinks of Active Power and Interaction with Sources and Sinks of Reactive Power
It is helpful to focus on sources and sinks of reactive power when studying the voltage levels in an AC power grid. It is known that voltage levels in an AC power grid are closely related to absorption or injection of reactive power. The control of the AC power grid""s voltage is in fact closely related to the control of reactive power. An injection of reactive power at a bus in the AC power grid will generally increase the voltage in the nearby surrounding grid.
The traditional most influential sources and sinks of reactive power in power systems include the following:
Overhead AC lines generate reactive power under light load since their production due to the line capacitance exceeds the reactive losses in the line. Lines absorb more reactive power than they produce under heavy load.
Underground AC Cables produce reactive power since because of their high capacitance, the reactive losses never exceed the production, under normal operating conditions. This, in turn, limits the length of the transmission cable for standard frequency AC and favors of DC or LF AC.
Transformers absorb reactive power because of the reactive losses.
Shunt capacitors generate reactive power.
Shunt reactors absorb reactive power.
Synchronous compensators (condensers) and static VAr compensators can absorb or generate reactive power depending on the need of the surrounding part of the power grid, i.e., in a regional part of a TandD grid where reactive power is distributed and collected (intentionally allocated), without any major drawbacks regarding voltage levels and power quality.
Series capacitors are connected in series in highly loaded lines and thereby increase their production of reactive power.
Loads seen from the transmission system are usually inductive. So, they absorb reactive power.
Synchronous machines, SM, can generate and consume reactive power as well as active power, whereby they are called upon to be operated as generators or motors depending on their production or consumption of active power.
These devices form control measures which are generally classified as either static or dynamic. Static components such as shunt capacitors and reactors can be regulated in fixed discrete steps and with some time-delay and therefore cannot be used to improve system response to fast phenomena related to transient dynamics in power systems. They are, however, very reliable and a cost-effective way of reactive compensation for steady-state optimization as long as they are installed in moderate amounts.
On the other hand, dynamic compensation devices such as static VAr compensators, synchronous condensers and generators are more expensive, but can be controlled continuously and be used to improve transient response of the power system because of their short response time. Dynamic compensation is however more expensive per MVAr of compensation. Often, reactive demand from loads close to generation areas is supplied by generators and by the cheaper static devices such as shunt capacitor banks and reactors in load areas far from generators. It is however, not unusual to install capacitor banks in combination with dynamic devices to increase the control range of these devices. OLTC, On-Load Tap Changers, on power transformers are very cost-efficient tools for voltage control.
Reactive power compensators were, in earlier days, mainly based on the above-mentioned synchronous machines. They are fundamental as actuators for a stable operation of power systems. More recently, compensation is often provided by thyristor-controlled capacitors. SVC is an acronym for Static Var Compensators used during approximately 20 years in the transmission area. Another device is the Advanced Static Var Compensator, earlier often shortened as AdvSVC, but also called STATCON, and more recently standardized by CIGRÉ/IEEE to STATCOM an acronym for Static (Synchronous) Compensator.
AC power flow in the AC power grids is, in modern electrical power engineering, closely connected to HVDC, High Voltage transmission DC links, and to RandD on FACTS, Flexible AC Transmission System, an acronym and a family name for several thyristor-based devices. FACTS includes at least:
STATCOM Static Synchronous Compensator (=AdvSVC, Advanced Static VAr compensator)
TCSC Thyristor Controlled Series Capacitor
TCPAR Thyristor Controlled Phase-Angle Regulator
UPFC Unified Power Flow Controller
MPTC Multi-level Power Transfer Controller
SVS Synchronous Voltage Source
SSSC Static Synchronous Series Compensator
TCPST Thyristor Controlled Phase Shifting Transformer
PST Phase Shifting Transformer (strictly written, PST is a competitor to FACTS).
The Unified Power Flow Controller, UPFC, consists in principle of a Static Synchronous Compensator, STATCOM, as the shunt element combined with a Static Synchronous Series Compensator, SSSC.
The Power Flow Controller (which includes a shunt xM, or SM and a series xM or SM) depicted inside the coactive converter, FIG. 4, for example, is a competitor to UPFC and preferably embodied with two cable-based rotating machines as described in PCT Publication WO 99/29008, entitled POWER FLOW CONTROL, incorporated herein by reference, as a shunt machine and a series machine. When implemented, this would become another example of a type of FACTS device.
When improving the performance of an AC power grid for renewables, it is necessary to deal not only with sources and sinks of reactive power, HVDC and FACTS, but also with sources and sinks of active power and their interaction with sources and sinks of reactive power. A similar list of traditional sources and sinks of active power could be compiled. It is thereby only necessary to deal with those sources and sinks that are important to electric power that is applied to an electric grid after being generated from renewables in order to make that electric power as commercially valuable and fungible as electric power produced by traditional sources and sold on a power exchange. The list of sources and sinks of active power and their interaction with sources and sinks of reactive power associated to renewables is thus limited to the following:
Synchronous machines, SM, can generate and consume reactive power as well as active power, whereby they are called to be operating as generators or motors depending on their production or consumption of active power. SM also stores energy in their moment of inertia, J, often associated with the technical term inertia constant, H. H in the range of 3 to 5 seconds means that the stored energy is equivalent to nominal power during 3-5 s. The so-called swing equation discussed below, describes the pendulum mode interactions between the power grid and synchronous electrical machines.
The swing equation for a single generator can be written as:                     2        ⁢        H                    ω        0              ⁢                            ⅆ          2                ⁢        δ                    ⅆ                  t          2                      =            P      mech        -                  P                  e          ,          max                    ⁢      sin      ⁢              xe2x80x83            ⁢      δ      
where
Pmech is mechanical input (in p.u.; per unit)
Pe,max is maximum electrical output (in p.u.)
H is inertia constant for generator (in MWs/MVA) defined as H=0.5xc2x7Jxc2x7(xcfx89/p)2/(Sbase) (all in S.I.)
xcex4 is rotor angle (in electrical radians)
t is time (in seconds)
The electrical power output from the electrical machine to the power system can, based on the above swing equation, be written as:       P    e    =                              E          ⁢                      xe2x80x83                    ⁢          V                          X          T                    ⁢      sin      ⁢              xe2x80x83            ⁢      δ        =                  P                  e          ,          max                    ⁢      sin      ⁢              xe2x80x83            ⁢      δ      
with emf E, rotor (load) angle xcex4 as the angle between the bus voltage V and the voltage behind the synchronous reactance E, the reactance XT as the sum of the reactances of the transient reactance of the generator and the reactance of the transmission line, and bus voltage V at the far end of the line, i.e.,
XT=Xxe2x80x2d+Xl 
and the maximum electrical output is:       P          e      ,      max        =            E      ⁢              xe2x80x83            ⁢      V              X      T      
Although not a direct source or sink of active power, synchronous compensators (condensers) (i.e., a special type of the family of synchronous machines, SM) can not only absorb or generate reactive power but also store energy in their moment of inertia, J, often associated with the technical term inertia constant, H and thus are a source of kinetic energy. This kinetic energy may thus be harnessed to be used as a source or sink for generating/sinking active power. It is known that for synchronous compensators the swing equation""s Pmech=0.
Asynchronous machines, AM, can generate and consume active power but consume only reactive power, and are called to be operating as generators or motors depending on their production or consumption of active power. AMs also store energy in their moment of inertia, J, often associated with a technical term xe2x80x9cstart-up time constantxe2x80x9d, but is loosely connected to the grid frequency due to the asynchronous principle.
STATCOM (AdvSVC) is sometimes embodied with a DC power source, e.g., a fuel-cell, a prime mover, or, quite simply, an accumulator (a battery) on its DC-link. STATCOM alone has very limited energy stored in its DC-side capacitor compared to synchronous machines.
Power electronic converters based on IGBTs, GTOs, SCRs and the like, operating in inverting or rectifying mode. Those converters that are mains-commutated or machine-commutated (embodied with SCRs) are characterized by a, more or less, closed relation between active and reactive power while those that are self-commutated (embodied with IGBTs or GTOs) provide a large freedom regarding active and reactive power. Power electronic converters transfer more or less the electrical power immediately from its input to its output. They provide voltage transformation so that AC and DC quantities are to some extent interlaced but those converters that are based on IGBTs and GTOs provide a large freedom regarding transformation ratios and especially dynamics of the voltages"" magnitudes and phase angles.
As recognized by the present inventors, both rotating machines and power electronic converters are examples of sources and sinks that are important to electric power that is applied to an electric grid after being generated from renewables in order to make that electric power as commercially valuable and fungible as electric power produced by traditional sources sold on a power exchange.
In conclusion, regarding the background art, it is fitting to say that renewable power plants are today burdened by xe2x80x9cweaknessxe2x80x9d and xe2x80x9cvariabilityxe2x80x9d. Therefore, they do not offer power grid operators the voltage stiffness, fault handling and type of planned control that the power grid operators and owners are furnished with by those owning and operating traditional power plants, which for decades have produced commercially fungible and reliable power during normal operation as well as during faults.
On the other hand, renewables like wind power and solar electric power, are generally well recognized as environmentally friendly types of power, but not as commercially valuable or fungible as other types of electricity such as that generated by traditional sources like fossil fuel power plants, hydroelectric plants and nuclear plants.
The present description of the invention is not intended to be limited to the discussion in the following few paragraphs in this section, but rather is a synopsis of selected facets of the present invention. A more complete understanding of the present invention will be obtained in view of the teachings throughout this document. Nevertheless, an object of the present invention is to address the above-identified and other shortcomings of conventional systems and apparatuses using renewable technology.
Widespread Use of Renewables Gives Rise to Need for Converters
Traditional rotating AC machines, like synchronous machines, SM, and their traditional control systems, used for power plant facilities with traditional energy sources, provide stiffness and planned control, but are not sufficient when renewable facilities are employed on a large scale in the AC power grid. The other traditional rotating AC machines, asynchronous machines, AM are mainly used as motors, but from a cost point-of-view are used as generators coupled to e.g., wind turbines.
Renewable facilities are burdened by xe2x80x9cweaknessxe2x80x9d and xe2x80x9cvariabilityxe2x80x9d, thus not offering AC power grid operators the (voltage) stiffness and type of planned control that the power grid operators and owners possess with those who have produced commercially reliable power for decades during normal operation as well as during faults. These burdens result in new demands for rotating AC machines and their control systems to enhance electric power produced by renewable facilities, like wind turbine plants, solar electric power plants and the like, so as to make that electric power as commercially valuable and fungible as electric power produced by traditional sources, e.g., fossil fuel power plants, hydroelectric plants, nuclear plants and the like. The demands are:
reactive and/or active power control
adding/subtracting active power, such as with a prime mover, moment-of-inertia, and virtual energy storage
adding/subtracting reactive power
electromechanical conversion
power electronic conversion (e.g., SCR, IGBT, GTO)
reducing the low order harmonics pollution from the power electronic conversion
symmetrizing the power grid
supplying short-circuit power during faults
start-up of the power grid after major faults
sometimes frequency conversion
These demands can be summarized as demands to xe2x80x9cprimexe2x80x9d power from renewable facilities.
A new class of converter according to the present invention includes controllable/coordinatable, constant/variable-frequency output, converters that include traditional and innovative combinations of:
1. electromagnetic, electromechanical converters (i.e., rotating machines) configured for electromechanical conversion;
2. power electronic converters for reactive and/or active power control, sometimes including frequency conversion;
3. at least one digital processor with a computer program product and communication and control mechanism; and
4. inputs from and outputs to other digital processors with computer program products and mechanisms for communication and control.
The two first demands are fulfilled regarding rotating AC machines, to a higher degree by the constant-frequency output, variable/variably speed rotating AC machine, (xM), but to a lower degree by the traditional synchronous machines, SM, which are operating with constant-frequency output, constant speed.
One inventive aspect of the present invention is a coordinated and controlled intercommunication and operation of power engineering equipment and converters, e.g., rotating AC machines, power electronic converters and transformers as well as power grids in order to enhance electric power produced by renewable facilities. As viewed from outside the present invention, i.e., viewed from the power grid and its stakeholders"" perspective, renewable facilities are seen as stiff and produce power that is as commercially valuable and fungible as electric power produced by traditional plants such as fossil fuel power plants, hydroelectric plants, nuclear plants and the like. xMs and SMs fulfill the demands, when properly employed, which the present inventors recognize can be accomplished with the coordination of system assets in a continuing, or at least periodic, manner.
An intelligent coactive converter is connected to a standard frequency AC power grid and enhances electric power produced by a renewable facility. A constant-frequency output, rotating AC machine, a digital processor with a computer program product and, mechanism for communication and control (together forming a controller) are included in the intelligent coactive converter. The digital processor and the computer program product control, via the communication mechanism, primarily the constant-frequency output, rotating AC machine and a number of rotating AC machines in the renewable facility, to prime, regarding voltage stiffness and power variability, the electric power produced by the at least one renewable facility when feeding the power to the standard frequency AC power grid.
Renewables, when comprising a large percentage (e.g., 20% or more of power generation sources) cannot be allowed to be the cause of a devastating deficit of power production delivered to a power grid. Electric power from renewables when used in a large scale, needs to be as commercially valuable and fungible as electric power produced by traditional sources sold on a power exchange under at least the following conditions:
The coldest day in Denmark is strongly associated with a peak load within the Danish Eltra Grid.
The Grid""s connections to the surrounding Grids are then often stressed to their limits.
Conventional (security assessment) criteria like N-1 and N-2 security constraints (contingencies) are necessary to analyze bottlenecks and similar constraints.
Voltage stiffness in the AC power grid""s nodes is critical.
Similar deficits appear in other grids like those in the US, e.g., the grid in California most likely on the hottest day, in which there is a peak load.
One object of the present invention is to provide systems, methods, rotating machines, and computer program product that convert electrical power generated from renewables like wind power, solar electric power, and the like, into premier power, i.e., as commercially valuable or fungible as traditional types of electricity.
Another object of the invention is to overcome the xe2x80x9cweaknessxe2x80x9d and xe2x80x9cvariabilityxe2x80x9d that are associated with renewables. Weakness and variability hinder AC power grids"" system operators from possessing the voltage stiffness and the type of planned control that they and power grid equipment owners enjoy from those who have for more than a century commercially and reliably produced power using non-renewable sources. With renewables, there is variability during normal operation and there is weakness during faults where the level of short circuit power and its distribution is important for the voltage stiffness.
Another object of the invention is to use the system hardware and software resources for more than one task, for best cost-effectiveness.
An example of this is to use the moment of inertia normally inherent in the wind turbines and the rotating electrical machines to reduce the influence from weakness and variability.
Another example is to use the current overload capabilities inherent in the rotating electrical machines to provide short-term short circuit power, by using the inherent adiabatic character of windings"" copper temperature rise, to provide fault handling, system protection and voltage stiffness.
Still another example is then to use the rotating shaft of xMs or SMs to assure deliveries of sold energy in case of lack of transmission capability by clutching prime movers.
In one embodiment, an xM machine is employed as part of xe2x80x9ca coactive converterxe2x80x9d to ensure that steady, fixed frequency power is reliably applied to the power grid, thus eliminating problems with weakness, voltage stiffness and variability. In selected embodiments, the invention uses a prime mover, P.M., as an active power source to reduce and/or eliminate the variability drawback when enhancing electric power produced by renewable facilities so as to make that electric power as commercially valuable and fungible as electric power produced by traditional sources.
Embodiments according to the present invention include always independent of xM or SM, a computer and a computer program product which perpetually models and controls the actual converters"" operational status, such as:
1. air-gap-flux {overscore ("PHgr")}xcex4, stator currents {overscore (I)}S, rotor currents {overscore (I)}R, load angle xcex4, power factor cos xcfx86 and the like in the actual rotating machines,
2. firing angles xcex1, respect angles xcex3 and the like in the actual power electronic converters,
3. as well as voltage control and similar features associated with reactive power, and
4. the active power balance and inherentxe2x80x94by computers controllablexe2x80x94torque relations to create and keep reasonably small margins for best cost-effectiveness and best availability.
The TandD capacity to incorporate renewables"" power and transfer all types of active power can be assured by keeping the voltage stiffness, i.e., a grid voltage control via a proper (re)active power distribution. Renewables in the Eltra Grid, for instance, e.g., can therefore be primed (via virtual energy storage) with hydro power from Norway and/or Sweden and with distributed/decentralized prime movers (P.M.+xM or P.M.+SM) and other sources.
It is another object to reuse system resources for more than one task, i.e., to use xe2x80x9ctwo-in-onexe2x80x9d, for best cost-effectiveness, e.g., to utilize inherent moment of inertia, inherent current overload capability, etc., as solutions that serve, for a limited time period at each fault or at repetitive stress occasion.
These solutions are proposed to ensure continuous operation of the renewables plants as well as traditional sources, e.g., fossil fuel power plants, hydroelectric plants, nuclear plants and the like, i.e., to overcome risks for devastating disturbances and/or power imbalance.